Portable transponders (hereafter referred to simply as “transponders”) such as radio frequency identification (RFID) transponders, usually comprise one or more semiconductor chips having logic and/or data handling capabilities, attached to one or more interface devices, such as an antenna. A transponder (which may also be referred to as a “tag”) can communicate with external devices, such as interrogators and, via such interrogators, with supporting infrastructure, for example application middleware. Typically, a transponder can transmit or respond with one or more identities from a global numbering scheme. A transponder may also include memory for storing fixed or updatable data and/or sensors for detecting or measuring temperature, pressure etc.
Commonly, transponders are used to identify objects to which they are physically attached. Objects identified or tracked through transponders are hereafter called “tagged objects”. Transponders on tagged objects can be used to determine the location of objects and/or monitor the environmental variables around such objects, for example temperature or pressure. Advanced transponders may incorporate actuators providing tagged objects with robotic or other capabilities.
Transponders communicate wirelessly with interrogators (also known as “readers” or “base stations”) typically via radio waves. In some systems, the interrogation medium need not be electromagnetic, but can be optical and/or acoustic. The interrogation range varies from few millimeters to several meters depending on the type of transponder and reader, frequency, media, antenna, interference and other factors.
Interrogators can, in turn, be connected to a network of other interrogators and computers running appropriate supporting software. An RFID system typically includes at least one interrogator and at least one transponder.
Transponders may be passive, which means that they are energised through electric or electromagnetic induction by the interrogation signal of the interrogator, or active, which means that they are energised by an internal power source, such a battery. Passive transponders can only operate within the interrogation field of an interrogator. Arrival of a transponder in an interrogation field is usually referred to as “energising” the transponder. Passive transponders are described in U.S. Pat. No. 3,713,148 A.
The use of RFID systems is becoming widespread. For example, cheap transponders are used to identify pallets, cases and units of fast moving consumer goods (FMCGs). RFID systems are also employed to track assets in a variety of fields such as manufacturing, logistics and distribution, amusement, rental and leasing, and are used in factories to manage conveyor belts, in airports to track baggage, and in retail to track groceries. Leading manufacturers, distributors and retailers are promoting the usage of transponders to replace barcode-based product identification procedures and so improve the visibility of their stock and automate their operations. RFID is also an environmentally-friendly technology. For example, RFID tags can help improve management of supply chains of perishable goods and so reduce the amount of perishable goods thrown away as waste. RFID tags can also be used in recycling and the re-use of packaging. RFID tag can even be used to tag trees and help to prevent illegal logging.
To operate, RFID systems require transponders and interrogators to communicate. Communication takes place using standard frequencies, protocols, procedures and numbering schemas. Recent years have seen a variety of groups defining standards and regulating the use of RFID, including: International Organization for Standardization (ISO), International Electrotechnical Commission (IEC), ASTM International, DASH7 Alliance, and EPCglobal. Examples of standard wireless protocols for RFID systems are ISO 14443, ISO 15693, ISO/IEC 18000 Parts 2, 3, 4, 6, 6C and 7, ISO 18185 and EPC™ Gen2.
Usually, transponders and interrogators communicate in both directions and behave according to standard wireless communication protocols which, among other technical characteristics, specify: (a) a set of valid commands and parameters to be transmitted by an interrogator and (b) a set of responses and actions to those commands by transponders. Among other functions, interrogator commands and their respective transponder responses can allow interrogators to:                1. Individually identify transponders from an in-range population, a process usually referred to as “inventorying”. Normally, commands used for inventorying do not address a specific transponder within the population, but a specific subset or the entire population. Commands aimed at more than one transponder are hereafter referred to as “collective commands”.        2. Address specific transponders individually and so upload or download data to or from a specific transponder or change its security level. For this, existing protocols usually employ a temporary identification number (commonly and hereinafter referred to as a “handle”). Commands addressing a specific transponder are referred to as “individual commands” or (as used in some standard protocols) “access commands”.        
Usually, transponders work like a state machine, changing their operational status according to the commands received from the interrogator and as defined by their working protocol. For example, EPC Gen2 transponders have 7 general statuses, namely Ready, Arbitrate, Reply, Acknowledge, Open, Secure and Killed, and other status-defining features called session and select flags. Some statuses are used for inventorying (e.g. in EPC Gen2, Ready, Arbitrate, Reply and Acknowledge) and others for working with specific transponders using individual commands (e.g. in EPC Gen2, Open and Secure). In this example, the operational status of an EPC Gen2 transponder is defined by the combination of its general status, the value of all its inventory flags, and other status-defining features, for example the current inventory session.
Normally, all in-range transponders simultaneously listen to the interrogator. Transponders do not hear each other's responses and usually only the interrogator can hear transponder responses. Because of this, transponders cannot coordinate their responses so may reply simultaneously to collective commands, an undesired behaviour known as “collision”. In conventional RFID systems, collision is unavoidable because the interrogator does not know the identities of newly-arriving transponders (hereinafter referred to as “unidentified transponders”), and therefore cannot address them individually. This limitation has been addressed in the past using an intelligent network which predicts the likely identity of incoming transponders per each interrogator and reference is made to GB 2 437 347 B. In other RFID systems, existing RFID protocols usually include anti-collision mechanisms involving selective addressing of transponder sub-populations or the use of randomly delayed responses, for example implementing a random number generator fed to a decreasing counter. Reference is made to CN 101359361 A, US 2008 180220 A, CN 101256617 A, US 2004 140884 A, WO 02 41650 A, TW 399190 B, KR 2010 0011711 A. Such mainstream standards as the ISO/IEC 18000-6C and EPC Gen2 define advanced anti-collision mechanisms and feature flags to differentiate identified from non-inventoried (unidentified) transponders, even providing for multiple sessions where various interrogators can independently inventory transponders within the intersection of their reading ranges. Furthermore, some standards also allow the selection of transponder population subsets through their data contents, including identities such as the standardised Electronic Product Code (EPC™), using “Select” commands, the nesting of which allows intersections and unions of matching or non-matching subsets.
However, research has shown that existing anti-collision mechanisms can suffer important performance limitations when dealing with large transponder populations, mostly due to the exponential degradation of these algorithms as transponder density increases. Example of such research is “Performance Benchmarks for Passive UHF RFID Tags” by K. M. Ramakrishnan and D. D. Deavours, Proceedings of the 13th GI/ITG Conference on Measurement, Modelling, and Evaluation of Computer and Communication Systems, Nuremberg, Germany, pp. 137-154 (2006). For instance, anti-collision mechanisms based on the selective addressing of transponders require interrogators to accurately estimate the number of in-range unidentified transponders, which is not always possible, and to issue a number of selecting commands the processing of which by transponders is slow, cumbersome and unreliable. Anti-collision mechanisms based on randomly delayed responses also require an accurate estimation of the in-range population, the size of which conditions the optimal spread of the random function.
Moreover, conventional anti-collision mechanisms perform particularly poorly when inventorying moving populations due to limitations as to the prioritisation of unidentified transponders. Unidentified transponders are all treated the same and therefore interrogators tend to miss transponders moving in or out of range while inventorying other segments of the population. This translates in a very poor tracking performance in applications where tagged objects move in last in, first out (LIFO) fashion (LIFO applications), for example as with the replenishment of shelves of fast-moving durable products.
The limitations of conventional RFID systems in dealing with large or moving populations are the consequence of mainstream protocols lacking collective commands that allow the prioritisation of transponders by functional criteria other than transponder identity or data, and inventory status. For example, conventional protocols offer no commands to address unidentified transponders by such other relevant functional criteria as energising time, arrival order, distance from the interrogator, or overall operational status. In high-performance applications, particularly those with high transponder density or mobility, or with LIFO mobility, it is desirable to have the facility to address finer subsets of an in-range population of transponders.